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An Investigation of the Selenium Concentration in Thenardite

Efflorescences on Mancos Shale, Western Colorado

Final Report

Prepared for

Ruth Powell Hutchins Water Center

Colorado Mesa University

Rachael C. Lohse & William C. Hood

Department of Physical and Environmental Sciences

Colorado Mesa University

May 24, 2018

Accomplishments

This is the final report on the accomplishments of the grant “An Investigation of the Selenium

Concentration in Thenardite Efflorescences on Mancos Shale, Western Colorado.” The major

accomplishments were:

We collected 106 samples of the white efflorescences that occur scattered about on the Mancos

Shale in Mesa, Delta and Montrose Counties, western Colorado.

In the laboratory, a portion of each sample was purified to remove clays and other particulate

impurities. The purified material was pressed into pellets and analyzed for selenium by x-ray

fluorescence.

Seven samples were sent to a certified laboratory to give us standards with which to analyze the

remaining samples by x-ray fluorescence. Upon establishing a relationship between the x-ray

response and selenium concentration, the selenium concentrations in the remaining samples were

calculated.

Selenium in the efflorescences ranged from less than one part per million to 441 ppm (by

weight).

Experiments were conducted to see if the selenium in the easily soluble efflorescences could be

incorporated into gypsum, which is much less soluble. Some selenium was trapped, but most

was not, suggesting that the selenium in the efflorescences is not in the form of selenate (SeO4)-2.

Portions of the original material from each of the 106 samples were analyzed by x-ray diffraction

to determine the mineralogy. By far the most abundant mineral was thenardite (Na2SO4)

although minor amounts of gypsum (CaSO4.2H2O) and bloedite (Na2Mg(SO4)2

.4H2O) also

occur. Bloedite was the major phase in one sample.

The results of the project were presented by Rachael at the CMU Student Showcase and then

again to the Grand Junction Geological Society at their April, 2017 meeting. Rachael won a

small cash award for her poster at the GJGS meeting.

An abstract of the work was submitted to the Rocky Mountain Section of the Geological Society

of America for consideration for presentation at their May meeting. The abstract was accepted

and is published on the GSA website (Lohse and Hood, 2018).

We presented a slightly revised version of the poster at the RMS-GSA meeting in Flagstaff, AZ,

on May 15, 2017. A photograph of the poster and authors is below (Fig. 1).

Figure 1. Rachael Lohse and William Hood with their poster at the RMS-GSA meeting in

Flagstaff, AZ on May 15, 2018.

Introduction

The presence of selenium in rocks, including rock units that are equivalent to the Mancos

Shale, has been known for decades (Howard, 1969a,b; Rapp and Patraw, 1966). In 1982, when

the mortality of fish and birds and the lowered reproductive success for birds in the Kesterson

Wildlife Refuge in California was traced to selenium in water (National Research Council

Committee, 1989). The source of the selenium was then traced to irrigation. This led to other

studies that identified selenium problems in the Green River drainage in Utah (Stephens et al.,

1988) and in the Colorado and Gunnison Basins in western Colorado in 1987-1988 (Butler, et al.,

1996). It was established that irrigation was contributing much of the selenium load to the water

of the western Colorado area (Wright and Butler, 1993) and work subsequently has concentrated

on ways to reduce the selenium load. Experimental projects involving bioremediation (Walker

and Golder Associates, 2010) and phytoremediation (Fisher, 2005) have been conducted and

much effort has gone into controlling underground seepage from irrigation structures (Butler,

2001). Recent investigations have examined the presence of selenium in weathered Mancos

Shale (Tuttle et al., 2014a, b) and in unweathered Mancos Shale in deep drill holes (Hood and

Cole, 2016).

This study emphasizes selenium because the metal is prevalent in western Colorado

surface waters, groundwater, and soils. Although small amounts of selenium are necessary for

health in both animals and man, high concentrations of selenium can cause harm to already

endangered species of native fish populations and aquatic ecosystems. Extreme selenium

toxicity cases can cause death to farm animals, wildlife and even man (Koller and Exon, 1986).

We observed many instances of selenium accumulator plants such as Stanleya pinnata (Princes

Plume) and species Astragalus growing in the vicinity of the efflorescences. However, the plant

most commonly observed in the efflorescences themselves was Hallogeton glomeratus, a salt-

tolerant, oxalate-accumulating plant that can be toxic to sheep (James and Cronin, 1974).

Selenium is naturally sourced from the Mancos Shale which covers significant portions

of western Colorado. The shale weathers easily and thus provides an optimal pathway for

selenium to enter the waters. As the shale weathers, the reduced forms of selenium in

selenopyrite or organic matter can be oxidized and released into solution (Mills et al., 2016). It

is most commonly leached by means of agricultural and domestic irrigation and leakage from

irrigation canals (Butler et al., 1996). In many places a white efflorescence occurs where

seepage from natural springs and seeps, irrigation ditches and canals reaches the surface and

evaporates. The efflorescence, commonly called “alkali” by local residents, is very high in

sodium and makes plant growth difficult. The springs and seeps in the area often contain high

quantities of selenium (Morrison et al., 2012). It seems probable that if ground and surface

waters contain excessive amounts of selenium, then the efflorescences derived from seepage of

ground water ought to contain selenium as well.

The main objective of the study was to determine the concentration of selenium in a

significant number of locations in western Colorado. The selenate ion is similar in geochemical

activity to the sulfate ion and can be incorporated into minerals such as gypsum, soluble salts

such as thenardite, and magnesium salts (Mills et al. 2016). Previous work by Hood

(unpublished) indicated that the efflorescence is largely the mineral thenardite (sodium sulfate)

so the incorporation of selenium into the efflorescence should be expected. However, little

research has been conducted to analyze the chemistry of the white efflorescence.

According to Tuttle et al. (2014a), natural undisturbed landscapes contribute only very

small amounts of selenium to the waterways of western Colorado. Precipitation from rain or

snow dissolves calcium and other ions on the land surface. The solutes percolate into the

subsurface to form gypsum and soil moisture containing sodium sulfate. The arid conditions

following the moisture event evaporate some of the soil moisture, concentrating the sodium

sulfate which is quite soluble. This portion of the state experiences around 9.5 to 10 inches of

precipitation annually. This is not enough precipitation to attribute to the large amount of

salinity in the rivers currently. Most of the contamination is derived from irrigated soils.

Irrigation accelerates the rate at which the sodium sulfate salts and selenate reaches the

waterways.

Methods

One-hundred and six sodium sulfate samples were collected in Mesa, Delta and Montrose

Counties, Colorado (Table 1 and Figure 2). Aerial imagery from Google Earth and ArcGIS

online provided guidance on locating the white efflorescences within the Mancos Shale. The

purest salts were collected at each sample site by scraping or scooping the material with an ash

shovel. A GPS reading was recorded at the sample location in lat./long. coordinates with a

Garmin eTrex and DeLorme Earthmate PN-60. Elevations of the sample sites were determined

by using a DEM (digital elevation model) of western Colorado in ArcGIS. The samples were

classified as: pure powder, mixed powder, and crust. Examples are shown in Figure 3.

Samples were processed and analyzed in the advanced geology and x-ray laboratory at

Colorado Mesa University following the field collection. The sodium sulfate was concentrated

by dissolving the crustal material in water. Soil and plant material were removed by funneling

the solution through coffee filters into glass bottles. A second filtration with Whatman No. 42

filter paper removed finer clay particles and ensured the solution consisted of purely dissolved

material. The second filtrate was collected in glass bowls that were placed in a convection oven

and dried at approximately 90 degrees Celsius. The recrystallized material in the glass bowls

was scraped out and ground to a powder by hand using a mortar and pestle. About two grams of

the powder was put into a Spex Sample Prep between two 13mm dies. A Carver hydraulic press

applied pressure of about 5000 psi to compress each sample into a pellet. The pellets were

analyzed for selenium using a Bruker Tracer III-SD energy dispersive x-ray fluorescence (XRF)

instrument optimized to detect selenium and elements with x-ray fluorescence characteristics

similar to selenium. The instrument produces a graph of photons detected over a period of time

vs energy level. The intensity of the selenium peak for each sample was calculated after

correction for background noise. Seven samples with a range of selenium peak intensities were

sent to Actlabs (Activation Laboratories Ltd.) in Ancaster, Ontario, Canada to be analyzed by

ICP-MS methodology. The results from these analyses compared with the corresponding XRF

peak intensities yielded a linear calibration plot which then allowed us to convert the selenium

peak heights into concentration in ppm (Figure 4).

To determine if the selenium in the efflorescences can be locked up in the mineral

gypsum, calcium chloride dihydrate was added to the set of samples with the highest

concentration of selenium. Calcium chloride is very soluble, quickly goes into solution and can

then react with the dissolved sulfate to form the much-less-soluble mineral, gypsum. The

chemical reaction is: Na2SO4 (aq.) + CaCl2·2H2O (s) → CaSO4·2H2O (s, gypsum) + 2NaCl2

(aq.). The liquid and precipitated gypsum were then centrifuged to concentrate the gypsum. The

gypsum was separated from the solution and dried. The solution was evaporated to dryness in

the oven, producing a product that is mostly salt (NaCl). The gypsum and salt were then pressed

into pellets and analyzed for selenium by XRF.

To determine the original mineralogy of the efflorescences, a portion of each sample was

sieved to collect the silt and clay fraction. This was done because it was recognized that the

efflorescences usually consisted of a fine white powder and so could be concentrated by a size

separation. For samples that consisted of a hard crust, we simply broke off a piece of crust and

pulverized it. The powders contained the efflorescence material plus quartz, clay and other

impurities. A portion of each powder was placed into a sample holder and the minerals

identified by x-ray diffraction.

Results

The results of the selenium analyses of the efflorescences are presented in Table 2 and

are summarized here. Twenty percent of the samples collected were between zero and 0.5 ppm.

Thirty-nine percent of the samples were between 0.5 ppm and 5 ppm. Twenty-five percent of the

samples were between 5 ppm and 50 ppm and fourteen percent were greater than 50 ppm (Figure

5). The maximum concentration found was 441 ppm in sample WP36. Maps showing the

sampling locations and the analytical results are shown in in Figures 6 – 10). Larger symbols

represent higher concentrations while small symbols represent lower concentrations. Low

concentrations were prevalent in the southernmost portion of the Montrose samples and graduate

to moderate/larger concentrations northward. The highest concentrations of selenium found in

the study were just north of the Montrose Regional Airport. This is highlighted in the Olathe

area result map. The land use in this area consists of agricultural and commercial development.

The Delta area contained two high concentrated samples in the center of the town in an area

surrounded by commercial development. The samples in the surrounding area recorded low to

moderate concentrations. Whitewater contained two localized ‘hot spots’ east of Highway 50

where concentrations were greater than 100 ppm. A small number of samples were collected in

Grand Junction. The highest Se values were within the city limits surrounded by commercial

development.

With regard to the gypsum precipitation experiment, the selenium concentrations in the

gypsum precipitate were compared selenium concentrations in the salt, which represents the

selenium not incorporated in the gypsum. Overall, the Se concentrations the gypsum precipitates

averaged 17 ppm whereas the concentration in the salt residue averaged 80 ppm.

Results of the mineralogy investigation are presented in Table 3. As expected from the

previous work by Hood (unpublished research) and the examination of 16 samples of the purified

efflorescence, the predominant mineral in the efflorescence samples is thenardite (sodium sulfate

– Na2SO4). Small amounts of gypsum (CaSO4.2H2O) and bloedite (Na2Mg(SO4)2.4H2O) were

found in several samples. Because the samples also contained quartz, clay minerals, calcite and

possibly other minerals, the x-ray diffraction patterns were difficult to interpret. We used a XRD

interpretation program, PDXL, supplied by the manufacturer of our instrument, to assist in the

interpretation. This program is adept at identifying single minerals but does not do a very good

job at identifying minerals in mixtures, especially when more than two minerals are present. We

had to resort to a very time-consuming procedure of eliminating the peaks for quartz and

thenardite and then trying to identify any other minerals that might be present based on the

remaining diffraction peaks. Small amounts of other sulfate minerals (mirabilite, hexahydrate,

vanthoffite, and leowite) were possibly present but could not be identified with certainty.

Because the Mancos Shale is a marine deposit, we made a specific search for diffraction peaks

related to the mineral halite (sodium chloride) but were unable to find any evidence that the

mineral is present.

Discussion

There were several limitations involved with the sample collection. Private property

prevented collecting samples in some circumstances. Verbal permission was granted in many

cases; however, land owners were unable to be contacted for some desirable locations.

Agricultural land use also posed a challenge to collecting samples, with livestock in the fields

where the efflorescence occurred. Impassible irrigation canals acted as a barrier to access to

some areas, including some with large amounts of efflorescence. The seasonality of the

efflorescence also limited the sample collection. We collected samples in July and August.

During the summer months, many of the areas characterized by efflorescence had shrunk

significantly in size or disappeared entirely. The patches of efflorescence will return and grow in

the winter months (Figure 11). Tuttle et al. (2014b) suggest that moisture evaporation takes

place in the dry winter months forming sulfate and selenate mineral. While this is true, it is not

the entire story. Grand Junction receives an average of 9.4 inches of precipitation per year, with

the highest amounts in September and October (about 1.2 inches) and the lowest in June (0.47

inches). February is also low. The city of Montrose is similar, with an average annual

precipitation of 10.19 inches (www.usaclimatedata.com). The significant factor is relative

humidity. In the winter months, relative humidity reaches a daily average of 70% in January. It

falls steadily down to 29% in June and stays below 40% through September

(www.currentresults.com/weather/). Many days during June, July and August the relative

humidity in the afternoons falls below 10% (personal observation). We hypothesize that the

efflorescences are slightly hydroscopic and when the humidity is high the particles tend to stick

together. During the hot, dry summer days with low humidity, they lose moisture and tend to

get blown away by the wind, except in areas where significant seepage keeps them moist.

The analytical results did not reveal a strong relationship between high selenium

concentration and sodium sulfate efflorescences in the Mancos Shale as originally hypothesized.

Many of the selenium concentrations are fairly low, less than one part per million. However,

there are localized ‘hot spots’ in each study area where selenium concentrations range from 100

to over 400 ppm. It was determined with the GIS data that the locations with the highest

concentrations are not in depressions or in areas characterized by low elevations. This shows

selenium concentrations have no apparent relationship with topography. The highest selenium

concentrations were not in the purest thenardite collected as we had hypothesized before

beginning the project. They were mainly in samples that contained powdered thenardite salts

mixed with soils or soils that exhibited thin crusts of thenardite.

The experiment that was conducted in an attempt to contain the selenium present in the

sodium sulfate by precipitating gypsum was partly successful. Although some of the selenium

was immobilized in gypsum, the results indicated that the selenium predominantly stayed in the

solution rather than being precipitated with the gypsum. The selenate ion (SeO4-2) and sulfate

(SO4-2) are similar in size, structure and charge, which allows the selenate ion to be incorporated

into gypsum. The fact that selenium stayed in solution indicates the majority of selenium within

the samples is in a form other than the selenate ion. Alternatives include the other three

oxidation states of selenium: selenite (SeO3-2), selenide (Se-2), and elemental Se. Selenide and

elemental Se are the least abundant in the Mancos Shale (Tuttle et al. 2014a). Therefore, we

http://www.usaclimatedata.com/

http://www.currentresults.com/weather/

hypothesize that selenite is likely present because dissolved selenium primarily exists as selenite

or selenate in the Mancos shale depending on redox conditions (Mills et al. 2016). Immobile

phases, especially adsorbed selenite (SeO3-2), happen to make up most of the selenium in Mancos

Shale soil (Tuttle et al. 2014a). Selenite (SeO3-2) is known to absorb into clays, hydrous ferric

oxides, or organic matter (Mills et al. 2016). We are currently unable to verify if all the selenate

(SeO4-2) entered the gypsum.

The mineralogy investigation confirmed that the main mineral component of the

efflorescences is thenardite (sodium sulfate). The total absence of any chloride mineral in the

efflorescences indicates that the sodium did not originate as trapped amounts of ancient sea

water. It must be present as part of the clay minerals in the shale, either as interlayer cations or

as exchange cations on clay surfaces. The sulfate in the efflorescences most probably comes

from the oxidation of pyrite (iron sulfide), which produces sulfuric acid as one of the products.

We suggest that the hydronium ion from the acid attacks the clay minerals in the shale, releasing

sodium in the process. Small amounts of magnesium are also released to give rise to the

bloedite.

Conclusions

White efflorescences that appear on the surface of Mancos Shale consist primarily of the

sodium sulfate mineral, thenardite, almost all of which contain detectible selenium. However,

the presence of an efflorescence is not necessarily an indication of a high selenium

concentration. About one-third of the efflorescences examined contained 1 ppm selenium or

lower but some contained as much as 441 ppm selenium. However, the study provides evidence

that high selenium concentrations are confined to certain locations in the study area. The maps

with graduated symbols produced in ArcGIS enhance the visualization of the distribution of the

selenium concentrations and should help guide future remediation efforts. The experiment

revealed that the selenium phases in the shale are likely in the forms of selenate (SeO4-2) and

selenite (SeO3-2).

Acknowledgements

We thank Colorado Mesa University’s Ruth Powell Hutchins Water Center for providing

financial support for this project. Sandra Hood helped with sample collection as well as

transporting us to and from the field sites. Colorado Mesa University chemistry department

provided the calcium chloride for the gypsum precipitation experiments.

References Cited Butler, D.L., 2001, Effects of Piping Irrigation Laterals on Selenium and Salt Loads, Montrose Arroyo Basin,

Western Colorado: U.S. Geological Survey Water-Resources Investigations Report 01–4204, 14 p.

Butler, D.L., Wright, W.G., Stewart, K.C., Osmundson, B.C., Krueger, R.P., and Crabtree, D.W., 1996, Detailed

study of selenium and other constituents in water, bottom sediment, soil, alfalfa, and biota associated with

irrigation drainage in the Uncompahgre Project Area and in the Grand Valley, west-central Colorado, 1991-

93: U.S. G. S. Water Resources Investigations Report 96-4138, 136 p.

Fisher, F.S., 2005, Uncompahgre Basin selenium phytoremediation: U.S. E.P.A. Grant WQC 01-00057 Final

Report, 150 p.

Hood, W. C. and R. D. Cole (2016) Geochemistry of the Mancos Shale as shown by the Fees Federal 2-6-8-101

well. Search and Discovery, Article 51357, 4 p.

Howard, J.H., 1969a, Geochemistry of selenium in earth-surface environments: in Abstracts for 1968: Geol. Soc.

America Spec. Paper 121, p. 446.

Howard, J.H., 1969b, Selenium in the Upper Cretaceous Niobrara Formation and Pierre Shale of south-central

Colorado, in Abstracts for 1968: Geol. Soc. America Spec. Paper 121, p. 606.

James, L. F., and Cronin, E. H., 1974, Management practices go limit death losses of sheep grazing on Halogeton-

infested range: Jour. Range Manage., 27, p. 424-426.

Koller, L. D. and Exon, J. H., 1986, The two faces of selenium – deficiency and toxicity – are similar in animals and

man: Can. Rev. Vet. Res., v. 50, 297-306.

Lohse, R. C.. and Hood, W. C., 2018, An investigation of the selenium concentration in thenardite efflorescences on

Mancos Shale, western Colorado: Geological Society of America Abstracts with Programs. Vol. 50, No. 5,

ISSN 0016-7592, doi: 10.1130/abs/2018RM-313456.

Mills, T.J., Mast, M.A, Thomas, J., & Keith, G., 2016, Controls on selenium distribution and mobilization in an

irrigated shallow groundwater system underlain by Mancos Shale, Uncompahgre River Basin, Colorado,

USA: Science of the Total Environment, v. 566-567, p. 1621-1631.

Morrison, S. J., Goodknight, C. S., Tigar, A. D., bush, R. P., and Gil, A., 2012, Naturally occurring contamination in

the Mancos Shale: Environ. Sci. and Technology, 46, p. 1379-1387

National Research Council Committee on Irrigation-Induced Water Quality Problems, 1989, Irrigation-Induced

Water Quality Problems. National Academy Press, 157 p.

Rapp, G. R., and Patraw, J.A., 1966, Distribution of selenium in the Niobrara Formation of the Black Hills region,

South Dakota (abstract); Geological Society of America Special Paper 87, p. 300.

Stephens, D.W., Waddell, B., and Miller, J., 1988, Reconnaissance investigation of water quality, bottom sediment,

and biota associated with irrigation drainage in the middle Green River Basin, Utah, 1986-87: U.S.

Geological Survey, Water Resources Investigations Report 88-401.

Tuttle, M. W., Fahy, J. W., Elliott, J. G., Grauch, R. I., & Stillings, L. L. (2014a) Contaminants from Cretaceous

Black Shale; I, Natural Weathering Processes Controlling Contaminant Cycling in Mancos Shale,

Southwestern United States, with Emphasis on Salinity and Selenium: Applied Geochemistry, Vol. 46, 01

July 2014, p. 57-71.

Tuttle, M. W., Fahy, J. W., Elliott, J. G., Grauch, R. I., & Stillings, L. L. (2014b) Contaminants from Cretaceous

Black Shale; II, Effect of Geology, Weathering, Climate, and Land Use on Salinity and Selenium Cycling,

Mancos Shale Landscapes, Southwestern United States: Applied Geochemistry, Vol. 46, p. 72-84.

Walker, R., and Golder Associates, Inc., 2010, Passive selenium bioreactor – pilot scale testing: U.S. Bureau of

Reclamation Science and Technology Program, Project 4414 Final Report.

Wright, W. G., and Butler, D.L., 1993, Distribution and mobilization of dissolved selenium in ground water of the

irrigated Grand and Uncompahgre Valleys, Western Colorado, in Allen, R.G., ed., Management of

irrigation and drainage systems: integrated perspectives: American Society of Civil Engineers, Reston, VA,

v. 1024, p. 770-777.

Figure 2

Figure 2. Map showing the locations of collection sites throughout western Colorado (imagery courtesy

of ArcGIS online).

Figure 3

Figure 3. Photographs of some examples of the white efflorescence. These photos were taken in the

laboratory. A. Purer form of sodium sulfate. B. Sodium sulfate mixed with soil. C. Thin sulfate crusts on

soil. D. Thick sodium sulfate crusts.

Figure 4

Figure 4. Calibration curve created by relating selenium values determined by ICP-OES at ACT Labs

plotted to XRF peak height after background correction.

Figure 5

Figure 5. Histogram showing the number of samples plotted with selenium concentration (ppm).

Figure 6

Figure 6. Selenium concentrations (ppm) in efflorescences collected in the Montrose, Colorado area

(imagery courtesy of ArcGIS online).

Figure 7

Figure 7. Selenium concentrations (ppm) in efflorescences in the vicinity of Olathe, Colorado (imagery

courtesy of ArcGIS online).

Figure 8

Figure 8. Selenium concentrations (ppm) in efflorescences collected in the vicinity of Delta, Colorado


Figure 9.

Figure 9. Selenium concentrations (ppm) in efflorescences collected in the Whitewater, Colorado area.


Figure 10.

Figure 10. Selenium concentrations (ppm) in efflorescences collected in the Grand Junction, Colorado

area. (imagery courtesy of ArcGIS online).

Figure 11.

Figure 11. Two views of sample collection area SP 102 and SP 106 taken close to the same spot. Top:

First sampling trip in March (SP 102). Bottom: Second sampling trip in August (SP 106).

Table 1. Locations of the samples used in this study. Sample Latitude Longitude Sample Latitude Longitude Sample Latitude Longitude

1 38.48814 -107.82924 45 38.56480 -107.95604 89 38.94237 -108.39381

2 38.50319 -107.81528 46 38.63224 -107.93019 90 38.60928 -107.92985

3 38.50334 -107.81528 47 38.64179 -107.92948 91 38.59090 -107.92447

4 38.50319 -107.83844 48 38.64335 -107.92739 92 38.55214 -107.89332

5 38.50009 -107.83846 49 38.64585 -107.92323 93 38.70957 -108.03425

6 38.47626 -107.80972 50 38.64605 -107.92276 94 38.71056 -108.02765

7 38.47592 -107.81844 51 38.64611 -107.92114 95 38.98460 -108.44644

8 38.45972 -107.80762 52 38.64510 -107.93006 96 38.98481 -108.44862

9 38.46205 -107.80208 53 38.63431 -107.93179 97 38.99489 -108.43671

10 38.46500 -107.80149 54 38.63467 -107.93284 98 38.96304 -108.37307

11 38.46867 -108.80174 55 38.63518 -107.93457 99 38.96237 -108.37173

12 38.48597 -107.80206 56 38.64168 -108.00726 100 39.07314 -108.57356

13 38.42138 -107.78721 57 38.69542 -108.13078 101 39.08105 -108.57720

14 38.42154 -107.78951 58 38.69745 -108.11430 102 39.07089 -108.53341

15 38.42157 -107.79704 59 38.70608 -107.99005 103 39.08733 -108.62637

16 38.42160 -107.80125 60 38.72630 -107.96880 104 39.09041 -108.62424

17 38.45022 -107.81545 61 38.75752 -107.94306 105 39.08975 -108.61989

18 38.45104 -107.81462 62 38.75893 -107.94500 106 39.07065 -108.53303

19 38.48871 -107.78506 63 38.73810 -108.13667 20 38.40407 -107.81713 64 38.72348 -108.13610 21 38.40743 -107.82010 65 38.79154 -107.97776 22 38.40771 -107.82050 66 38.79275 -107.97288 23 38.41005 -107.82153 67 38.80784 -107.89010 24 38.40973 -107.82169 68 38.80818 -107.88420 25 38.41102 -107.82022 69 38.82910 -107.88867 26 38.41083 -107.82023 70 38.82697 -107.89150 27 38.41031 -107.82075 71 38.78262 -107.88503 28 38.45160 -107.80280 72 38.78263 -107.88558 29 38.45761 -107.80057 73 38.78371 -107.88583 30 38.53841 -107.89604 74 38.78411 -107.88469 31 38.53684 -107.89583 75 38.79317 -107.83131 32 38.53670 -107.89557 76 38.74698 -108.07311 33 38.53603 -107.89557 77 38.74817 -108.07410 34 38.53630 -107.88917 78 38.97370 -108.43369 35 38.54128 -107.89844 79 38.97239 -108.43301 36 38.54167 -107.89880 80 38.97352 -108.43521 37 38.55201 -107.89333 81 38.96664 -108.42685 38 38.52960 -107.97166 82 38.96733 -108.42753 39 38.53041 -107.97260 83 38.96059 -108.42613 40 38.53031 -107.97124 84 38.96338 -108.42577

Table 2. Sample description and x-ray fluorescence analytical results.

Sample Number

Description Elevation (ft)

Se Corr. Peak Height

Se Conc. (ppm)

1 thin crust/mixed powder 6003 162 5.5

2 pure powder w/ plant material 6058 90 3.1

3 pure powder 6053 96 3.3


5 thin crust 5935 6244 212.9


7 mixed powder 6037 1984 67.7


9 thin crust/mixed powder (mainly dirt) 6104 407 13.9

10 thin crust 6103 300 10.2

11 mixed powder w/ plant material (mainly powder)

6124 906 30.9

12 pure powder 6150 143 4.9

13 pure powder 6224 49 1.7

14 mixed powder (saturated) 6205 55 1.9

15 mixed powder 6175 57 1.9

16 mixed powder/thin crusts 6159 0 0.0

17 thin crust w/ 'popcorn' appearance/mixed powder

6024 1959 66.8


19 mixed powder (mainly powder)/thin crust 6255 199 6.8


21 mixed powder/thin crust w/ plant material 6140 26 0.9


23 nearly pure powder w/ plant material 6125 21 0.7


6113 46 1.6


6110 10 0.3



6135 37 1.3




31 mixed powder (mainly salt) 5644 7785 265.5


5644 3441 117.3



35 mixed powder (mainly salt)/thin crusts 5632 2462 84.0





40 saturated crust 5572 5 0.2

41 pure powder/thick crust w/ plant material 5585 0 0.0

42 mixed powder/small pieces of crust w/ 'popcorn' appearance

5624 0 0.0

43 mixed powder (mainly dirt)/thin crusts 5620 0 0.0

44 mixed powder (mainly dirt)/thin crusts 5619 0 0.0

45 thin crusts (mainly dirt) 5591 629 21.4

46 mixed powder w/ plant material 5427 66 2.3


48 mixed powder (mainly salt) w/ plant material

5428 29 1.0

49 pure powder/thin crusts 5460 26 0.9


51 mixed powder (mainly salt)/thin crust 5472 21 0.7

52 pure powder 5399 17 0.6

53 mixed powder (mainly salt) 5420 158 5.4

54 mixed powder 5424 229 7.8


56 thick crusts (mainly salt) 5261 58 2.0

57 mixed powder/salty crusts w/ dirty plant material

5081 9 0.3

58 thick salty crusts w/ plant material 5124 3 0.1



61 mixed powder 5092 271 9.2


63 thin crust in dirt matrix 4902 19 0.6

64 'popcorn' crusts (mainly dirt) 5001 44 1.5

65 pure powder 5220 419 14.3

66 mixed powder (mainly dirt) 5231 161 5.5



69 thin crusts in dirt matrix 5995 123 4.2

70 thin crusts 5937 74 2.5

71 mixed powder/thin crusts w/ plant material 5142 0 0.0

72 pure powder 5143 58 2.0

73 pure powder w/ 'popcorn' crusts and plant material

5151 32 1.1


75 'popcorn' crusts (mainly dirt) in mixed powder matrix

5291 26 0.9

76 thin crusts/mixed powder 4935 828 28.2

77 thin crusts/mixed powder 4934 2400 81.8





82 thin crust/dirt matrix 4731 5 0.2

83 dirty crust (Sample A)/pure white crust (Sample B)

4732 3 0.1


85 mixed powder w/ plant material 4816 1 0.0

86 pure powder/pure crust w/ plant material 4875 29 1.0

87 mixed powder 4804 524 17.9



90 powder plus some dirt 5422 1132 38.6



93 mostly powder with some dirt 5134 0 0.0

94 orange crust on plant material 5134 55 1.9


96 nearly pure powder with some dirt 4653 1351 46.1



99 thick solid precipitate 4969 89 3.0

100 thin soil crust 4564 2216 75.6


102 pure powder 4611 48 1.6



105 thin soil crust 4534 227 7.7

106 thin soil crust 4611 21 0.7

.

Table 3. Results of sulfate mineral identification by x-ray diffraction.

A = abundant; the predominant sulfate phase. M = minor sulfate phase. T = trace amounts

Hex = Hexahydrite (MgSO4.6H2O); Mir = Mirabilite (Na2SO4.10H2O); V = Vanthoffite (Na6Mg(SO4)4);

L = Leowite (Na4Mg2(SO4)4.5H2O)

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1 A 38 A T 75 A M s

2 A 39 A M 76 ???????

3 A 40 A M Mir? 77 A s

4 A M 41 A T 78 A T

5 A T 42 A T 79 A T

6 A T 43 A 80 A T

7 A T 44 A M Mir? 81 A T

8 A T 45 A T 82 T A

9 A M 46 A T 83a A T

10 A T 47 A T 83b A T

11 A T 48 A T 84 A T

12 A T 49 A T t 85 A T

13 A M T H? 50 A T 86 A T

14 A 51 A T 87 A M

15 A T T L? 52 A t 88 A M M Hex?

16 A M 53 A T 89 A T

17 A T T 54 A T 90 A

18 A M 55 A 90a A T T

19 A M H? 56 A M 91 A T T

20 A T 57 A M 92 A T T

21 A T M 58 A M 93 A M

22 A T 59 A T 94a A M

23 A T 60 A T 94b A M

24 A T 61 A T 95 A T T

25 A T 62 A Y 96 A T

26 A M T V? 63 A 97 A M

27 A Mir? 64 A M 98 A M

28 A T T 65 A 99 A T

29 A 66 A M 100 A M

30 A M 67 A M Hex? 101 A

31 A T 68 A M s Hex? 102 A M M

32 A T 69 A 103 A M M

33 A 70 A Hex? 104a A T

34 A 71 A T 104b A

35 A T 72 A 105 A

36 A M 73 A 106 A M

37 A 74 A

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